Lower Squalene Epoxidase and Higher Scavenger Receptor Class B Type 1 Protein Levels Are Involved in Reduced Serum Cholesterol Levels in Stroke-Prone Spontaneously Hypertensive Rats.
Michihara A, Mido M, Matsuoka H, Mizutani Y.
Faculty of Pharmacy and Pharmaceutical Sciences, Fukuyama University, Fukuyama, Hiroshima 729-0292, Japan
A lower serum cholesterol level was recently shown to be one of the causes of stroke in an epidemiological study. Spontaneously hypertensive rats stroke-prone (SHRSP) have lower serum cholesterol levels than those in normotensive Wistar Kyoto rats (WKY). In order to elucidate the mechanisms responsible for the lower serum cholesterol levels in SHRSP, we herein determined whether the amounts of cholesterol biosynthetic enzymes or the receptor and transporter involved in cholesterol uptake and efflux in the liver were altered in SHRSP. When the mRNA levels of seven cholesterol biosynthetic enzymes measured by real-time PCR, farnesyl pyrophosphate synthase (FPS) and squalene epoxidase (SQE) in the liver of SHRSP were significantly lower than those in WKY. SQE protein levels were significantly reduced in tissues other than the brain of SHRSP. No significant differences were observed in low-density lipoprotein (LDL) receptor (uptake of serum LDL-cholesterol) or ATP-binding cassette transporter A1 (efflux of cholesterol from the liver/formation of high-density lipoprotein [HDL]) protein levels in the liver and testis between SHRSP and WKY, whereas scavenger receptor class B type 1 (SRB1: uptake of serum HDL-cholesterol) protein levels were higher in the livers of SHRSP. These results implicated the lower protein levels of SQE and higher protein levels of SRB1 in the liver in the reduced serum cholesterol levels in SHRSP.
Key words: squalene epoxidase, scavenger receptor class B type 1, stroke, cholesterol, liver
PMID : 26632180
An epidemiological study identified lower serum cholesterol levels as one of the causes of cerebral hemorrhage.1,2) SHRSP is a widely used animal model for hypertension and stroke.3,4) The serum cholesterol of this rat is also lower than that of normotensive WKY.5) Cholesterol is a major constituent of cellular membranes. Reductions in the cholesterol content in the cell have been shown to reduce the proliferation of cells, enhance cell membrane fluidity, and lead to fragile plasma membranes.6-8) Therefore, lower serum cholesterol is regarded as an important cause of cerebral hemorrhage.
We previously reported that lower mevalonate pyrophosphate decarboxylase (MPD), which is a cholesterol biosynthetic enzyme, is responsible for reduced serum cholesterol in SHRSP.9-11) However, it remains unclear whether cholesterol synthase downstream of MPD in SHRSP is reduced, or whether an increase in LDLreceptor (LDLR: uptake of LDL), an increase in SRB1 (uptake of HDL), or a decrease in Abca1 (efflux of cholesterol/formation of HDL) in the livers of SHRSP participates in reducing serum cholesterol contents.
In this study, we carried out the comparison of the mRNA levels of the cholesterol biosynthetic enzymes using liver between SHRSP and WKY. Next, we performed the comparison of the protein levels of the enzymes showed the reduction of mRNA levels. Furthermore, we examined the protein levels of LDLR, SRB1, and Abca1 in the tissue of SHRSP.
We showed that the lower protein levels of SQE as well as the higher protein levels of SRB1 played a role in reducing serum cholesterol levels in SHRSP (Fig. 1). The upregulation of SRB1 protein levels may occur in order to supplement decreases in cholesterol levels due to reductions in SQE and MPD protein levels in SHRSP livers. This may also have resulted in reductions in serum HDL-cholesterol in SHRSP. However, cholesterol contents in the liver were lower in SHRSP than in WKY, in spite of increases in the uptake of HDL-cholesterol from the serum to the liver due to the upregulation of SRB1. Therefore, the release of VLDL/LDL-cholesterol from the liver to the serum may have been decreased, leading to reductions in cholesterol contents in SHRSP livers. Reductions in the content of cholesterol in vascular endothelial cells in the SHRSP brain due to lower serum total cholesterol levels (VLDL/LDL/HDL) may lead to cerebral hemorrhage following a disruption in the blood-brain barrier.
The decrease of cholesterol contents was not caused in the testis of SHRSP, although the reduction of protein levels of SQE occurred in testis of SHRSP. No marked differences were observed in LDLR and Abca1 levels in the testis of SHRSP. These results suggested that decreases in cholesterol contents due to lower SQE protein levels in the testis of SHRSP were supplemented by a mechanism other than SQE, LDLR, and Abca1. If the accelerated uptake of cholesterol from the serum to tissues (other than by LDLR) or the suppressed efflux of cholesterol from tissues to the serum (other than by Abca1) occurs to supplement decreases in cholesterol contents due to reductions in SQE protein levels in tissues other than the brain of SHRSP, reductions in SQE protein levels in tissues is an important factor causing lower levels of serum cholesterol.
Highlights: We indicated that SQE protein levels were significantly reduced in tissues (lung, heart, pancreas, liver, spleen, kidney, and testis) other than the brain of SHRSP. Also, we found that SRB1 protein levels were higher in the livers of SHRSP.
- Iso H, Jacobs DR Jr, Wentworth D, Neaton JD, Cohen JD. Serum cholesterol levels and six-year mortality from stroke in 350,977 men screened for the multiple risk factor intervention trial.N. Engl. J. Med. 1989; 320: 904-910.
- Neaton JD, Blackburn H, Jacobs D, Kuller L, Lee DJ, Sherwin R, Shih J, Stamler J, Wentworth D. Serum cholesterol level and mortality findings for men screened in the Multiple Risk Factor Intervention Trial. Multiple Risk Factor Intervention Trial Research Group. Arch. Intern. Med. 1992; 152: 1490-1500.
- Okamoto K, Yamori Y, Nagaoka A. Establishment of the stroke-prone spontaneously hypertensive rat (SHR). Cir. Res. 1974; 34/35: 143-153.
- Yamori Y. in Gene-Environment Interaction in Common Diseases (Inoue, E. and Nishimura, H., eds.) 1977; pp.141-154, University of Tokyo Press, Tokyo
- Iritani N, Fukuda E, Nara Y, Yamori Y. Lipid metabolism in spontaneously hypertensive rats (SHR). Atherosclerosis. 1977; 28: 217-222.
- Bruckdorfer KR, Demel RA, De Gier J, and van Deenen LL. The effect of partial replacements of membrane cholesterol by other steroids on the osmotic fragility and glycerol permeability of erythrocytes. Biochim Biophys Acta. 1969; 183: 334-345.
- Morita I, Sato I, Ma L, Murota S. Enhancement of membrane fluidity in cholesterol-poor endothelial cells pre-treated with simvastatin. Endothelium. 1997; 5:107-113.
- Sato I, Ma L, Ikeda M, Morita I, Murota S. Simvastatin, a potent HMG-CoA reductase inhibitor, inhibits the proliferation of human and bovine endothelial cells in vitro. J. Atheroscler. Thromb.1998; 4: 102-106.
- Sawamura M, Nara Y, Yamori Y. Liver mevalonate 5-pyrophosphate decarboxylase is responsible for reduced serum cholesterol in stroke-prone spontaneously hypertensive rat. J. Biol. Chem.1992; 267: 6051-6055.
- Michihara A, Sawamura M, Nara Y, Ikeda K, Yamori Y. Lower mevalonate pyrophosphate decarboxylase activity is caused by the reduced amount of enzyme in stroke-prone spontaneously hypertensive rat. J. Biochem. 1998; 124: 40-44.
- Michihara A, Shimatani M, Akasaki K. Comparison of the gene expression levels of mevalonate pyrophosphate decarboxylase between stroke-prone spontaneously hypertensive and wistar Kyoto rats. J. Health Sci.2010; 56: 733-737.
Acknowledgment: This work was supported by Fukuyama University Grant for Academic Research Projects (GARP).